What happens to casualties and collisions when a 20mph scheme is implemented?

It’s a question that has been debated endlessly in discussion threads on this newsfeed. Which prompted us to see if we could shed any light on the subject.

The scope of our investigations

We focused on what happens to collision and/or casualty rates after a 20mph signed limit or engineered zone has been implemented. Did casualties/collisions increase or decrease, and how does the rate of any increase or decrease compare with that experienced across a wider area?

While some are mentioned in passing, we did not focus on any of the following:

• Severity of casualties (fatal, serious etc)

• Type of casualties (pedestrians, motorcyclists etc)

• Casualty/collision rates on neighbouring roads.

• Vehicle speeds

• Traffic volumes

• Emissions

• Public perception/support etc.

What data did we find?
It will not come as a shock to anyone who is involved with the 20mph issue to hear that there is a distinct lack of data available across the UK. And the data that is available is not collected and/or collated in a uniform manner – which makes analysis very difficult.

There are a number of areas where 20mph schemes have been introduced but ‘before’ and ‘after’ casualty/collision data is not available. There is data from a very well-established scheme in Hull but it dates back to 2002, which in our view makes it too old to be considered in this article. And the scheme in Edinburgh is at a relative early stage and as such the ‘after’ data is not yet available.

We did, however, manage to obtain ‘before and after’ data for 20mph schemes in the following 10 areas:

Some of the schemes are relatively small and as such the number of casualties/collisions is also small. This should be borne in mind when analyzing the data.

What did the data tell us?

• In five areas - London, Birmingham, Portsmouth, Newcastle and Gateshead – collisions and/or casualties fell at a significantly greater rate after a 20mph scheme was implemented, than would have otherwise been expected.
• In Warrington, while collisions fell at a significantly greater rate after the scheme was implemented, the number of casualties increased slightly.

• In Middlesbrough there was little or no significant difference between collisions/casualty rates in the 20mph scheme and the rest of the town/city.

• In Bristol the rate of collision reduction in the 20mph pilot areas was lower than across the city as a whole.

• In Hertfordshire and Derbyshire, while average collision rates in the 20mph schemes fell by 45% and 36% respectively, comparison with rates across the rest of the county is not possible (see below for more detail).

The following table summarises the findings. It shows the areas where collision reductions were higher in 20mph schemes than elsewhere; those where the reductions were lower; and those areas where it is not possible to compare. Best practice suggests that schemes should be evaluated over a minimum six-year period (three years before and three years after) in order to have sufficient data - three of the schemes do not meet this time period. The table also shows the difficulties of comparing the current evaluations of 20mph schemes.

Conclusions
• There is an urgent need for a national initiative to record and report in a uniform way data relating to 20mph schemes. Given the investment that is being made in this area, it is perhaps very surprising that this is not already taking place.

• The lack of clearly-presented data makes it very difficult to conclusively assess the impact of 20mph schemes with regard to casualties and collisions.

• Having said that, in half of the 20mph schemes we looked at, casualties and/or collisions were significantly lower than would otherwise have been expected – and in only one of the schemes were collisions higher than would otherwise have been expected.

• While not suggesting that the 20mph schemes are the only reason for these reductions, it is perhaps reasonable to assume they have played a part in this.

Results area by area

Here is a summary of the data for each area, along with links to full reports where these are available.

London (self-enforcing engineered zones)

Summary: a large-scale study over an extended period which concluded that “20 mph zones are effective measures for reducing road injuries and deaths”.

The researchers analysed Police STATS19 data for the period 1986-2006, to identify changes in counts of road injuries. Estimates of the effect of introducing 20 mph zones on casualties within those zones and in adjacent areas were adjusted for the underlying downward trend in traffic casualties.

The researchers concluded that the introduction of 20 mph zones was associated with a 41.9% reduction in road casualties, after adjustment for underlying time trends. The percentage reduction was greatest in younger children and greater for the category of killed or seriously injured casualties than for minor injuries. There was no evidence of casualty migration to areas adjacent to 20 mph zones, where casualties also fell slightly by an average of 8%.

Given the scale of the study, it was possible to remove data for three, four and five years before the introduction of the zones in order to account for any high casualty rates in site selection periods. The removal had little effect on the results of the casualty outcome and suggested that regression to the mean is not the explanation for the observed effects.

The general trend in casualties and collisions over time in London, an annual decline of 1.7%, was equivalent to a 15.8% reduction over 10 years or a 29.0% reduction over 20 years. Thus, in broad terms, the additional effect of the 20 mph zones was that of a step reduction in casualties and collisions by an amount that has taken over 20 years to achieve on roads without 20 mph zones.

Summary: mature, area-wide scheme under which collisions have reduced by 18.8% on the roads where a new 20mph limit has been implemented, compared with 10.3% across the rest of the city’s streets.

Portsmouth City Council was the first local authority in England to implement an extensive area-wide 20 mph speed limit scheme covering most of its residential roads which previously had a 30 mph speed limit. The scheme, which covers 94% of the city’s residential road length, was implemented in 2008.

Analysis has been undertaken of all injury collisions which occurred in Portsmouth for the three years before the 20mph scheme was implemented (2005-2007) and for the three years after (2009-2011). Collisions occurring in 2008 have been excluded as this was the implementation year.

All injury collisions were included to ensure a large sample size and to avoid any skewing of the data where severity levels might have increased with higher speeds.

The analysis shows that in the three-year period prior to the scheme there was a total of 505 collisions. The corresponding figure for the three-year period following implementation is 410 collisions, which equates to a reduction of 18.8%. Across the rest of Portsmouth’s roads, during the same period the number of collisions fell by 10.3% - from 1,618 to 1,451.

Warrington Borough Council carried out a 20mph pilot in three areas in the town where there was a history of higher casualties among vulnerable road user groups. The pilot ran for an 18-month period from February 2009 to August 2010.

In order to determine the true impact of the 20mph limits, the project team looked at the borough-wide collisions and casualty trend using the same ‘before’ and the ‘actual’ trial period. In percentage terms this showed an authority wide reduction of 11.9% in the number of collisions and 8.1% reduction in the number of casualties.

Taking the borough-wide reductions into account, the pilot resulted an overall reduction of 13.7 collisions (25%) over and above that which might have been expected – from 53.7 to 40. However, with regard to casualties there was an increase of three – from 59 to 62. The casualty increase was in one of the trial areas – the town centre. In the other two areas, casualties were slightly down.

Bristol (signed speed limits)

Summary: two pilots covering 500 roads and 30,000 households, in which collisions in the pilot areas reduced at a slower rate than across the city as a whole.

In the two years before the pilots there were 382 collisions across both areas. In the two years after implementation there were 364 collisions, which equates to a reduction of 4.7%.

With regard to collisions across Bristol, looking first at the South pilot, in the two years before the pilot there were 2,201 collisions across the city. The ‘after figure is 2,034 which equates to a reduction of 7.54%.

Looking at the East pilot period, the number of pre-pilot collisions was 1,981 and the number of post collisions was 1,784, which equates to a reduction of 9.9%.

It must be concluded, therefore, that the rate of collision reduction in the 20mph pilot areas was not as great as it was across the whole of the city.

The data for Newcastle and Gateshead led the authors to conclude that “whilst it should not be implied that the reduction in speed limits is the only cause, these statistics show that it is probable that these 20mph areas have helped to further reduce the number of collisions in these areas, over the reductions seen through the rest of the city in the same period”.

However, in Middlesbrough the 20mph schemes have yielded no collision reduction benefits, with identical reduction of 7% recorded in the 20mph areas and across the rest of the city.

Middlesbrough
There have been two phases of 20mph speed limit deployment in Middlesbrough; phase one ran March-June 2012 and phase two March-June 2013. The second phase has not yet been analysed.

Comparing the 14 months prior to implementation (Jan 2011 – Mar 2012) with the 14-month period post implementation (July 2012 – August 2013), the number of collisions in the new 20mph areas fell by 7%. However, the number of collisions right across Middlesbrough in the same period also fell by 7%.

A combined analysis of phases one and two is scheduled for 2014 when there will be at least one full year’s collision data available for each phase.

Newcastle

There have been six phases of 20 mph deployment in Newcastle, each containing several 20 mph zones that have been in force for at least a year up to the end of June 2012.

The data shows that overall collisions in these areas dropped by a yearly average of 25% following the introduction of the 20 mph zones.

The North East Regional Road Safety Resource says that “if collision reduction in 20 mph areas had matched the average reduction in Newcastle, we would have expected to see 21 fewer collisions per year between the before and after periods. However, we actually saw 59 fewer collisions per year, showing that there are 38 fewer collisions across the 20 mph areas each year than would probably have been the case if they had not been in place”.

Gateshead

In Gateshead, there are 21 20mph zones that have been in force for at least a year up to the end of March 2012.

Overall, there was an annual reduction of 7.5 collisions (13%) across all of the zones following their introduction. This equates to three fewer collisions per year than would be expected given the overall collision reductions across the rest of Gateshead.

Birmingham (engineered zones)

Summary: small sample, collisions and casualties both down, but comparison with pan-Birmingham casualty trend in same period is difficult.

As part of the Inner City Safety Demonstration Project, Birmingham City Council collected ‘before and after’ data in four areas where 20mph zones were introduced. The data relates to the three years prior to introduction, and three years post introduction. All of the data was collected during the period January 2005 – September 2012.

In total, across the four areas there were 44 collisions and 56 casualties during the pre-20mph period. The corresponding figures for post-20mph installation were 23 collisions and 30 casualties.

In terms of severity, there were no fatalities either before or after 20mph installation. Of the casualties in the pre-20mph period, four were serious and 52 were slight. The corresponding figures in the post-20mph period were two serious and 28 slight.

Drawing a direct comparison with collision/casualty reductions across Birmingham as a whole is not easy, because the 20mph trials were conducted at different times.

The reductions in collisions and casualties in the post-20mph period equate to 47% and 46% respectively. Across Birmingham, the number of collisions in the three-year period 2010-12 decreased by 35% compared with the period 2005-7.

However, only 11 of the sites have been operating for a minimum period of three years. Of the remaining six, three were only implemented in January 2013, and as such have been discounted from the analysis. A further three will reach three years in the first quarter of 2014, and we have adjusted the number of collisions to reflect this.

Across the 14 zones, the number of collisions in the three years prior to implementation of the 20mph zone was 30. The number of collisions in the three years post-implementation (including the adjustment referred to above) was 16.44, which represents a reduction of 45.2%.

Because the schemes were implemented at different times between December 2001 and March 2011, it is impossible to compare this with overall collision figures across Hertfordshire.

Derbyshire (engineered zones)

In Derbyshire, 26 20mph zones have been implemented between the years 2000 – 2012, in towns and villages across the county. We have excluded the two most recent schemes (in Matlock and Dronfield) on the grounds of insufficient ‘after’ data. Another scheme (Brimington) was introduced in November 2010, and while we do not quite have a full three years of data we have included this in the analysis.

In the three years prior to installation there were 106 collisions in areas where the zones were introduced. In the three years after installation the number of collisions was 68 which equates to a reduction of 36%.

As with Hertfordshire, because the schemes were implemented at different times over an extended period, it is impossible to compare this with overall collision figures from across Derbyshire.

I am not a fan of social control. 33% cut in your salary? It’s only a few pounds, and it will save lives.

Having done some analysis, 20mph limits work. Why wouldn’t they? Reduce speeds one third of the way to zero and you’d expect them to work. Reduce them all the way to zero and you remove road injuries altogether.

It takes suppressing between 100,000 and 300,000 drivers to save a pedestrian’s life. Outliers were Newcastle (35,000 drivers, with no fatalities in the last two years) and Bristol, where fatalities have increased in net terms.

The sacred cow of road safety remains intact. Not one council that I noticed even mentioned that pedestrians might somehow be involved in their own injury. It is the secret the government dare not mention. As ever, it’s left to the driver to be suppressed until even pedestrians can’t hurt themselves.

Analysis available shortly via email (andrew@andrew-mather.com) or Amazon unless Nick has some ‘document archive’ where I can leave it. In the meantime, here’s the results summary on Facebook Experience Counts
https://www.facebook.com/457909031052692/photos/a.457913327718929.1073741828.457909031052692/471871759656419/?type=1
Andrew Mather, Kent

This argument will - as is proving through this column - go on and on. Are there two sides? Or is it one side creating molehills and boosting them into mountains through selected 'data' in the name of safety? Is this not emotion overpowering commonsense?
Derek Reynolds

Nick
A right of reply, please. If you start with a presumption that slower traffic causes fewer casualties, then find that an intervention, say, reduces average speeds, then deduce through modelling or any other means that casualties will fall, you miss any possibility of identifying unintended consequences. I note than none of the authors of the London report had any safety or transport subject matter knowledge, so it is not surprising that they produced a statistical masterpiece, with health associations, but they did not appreciate the complexities of what happens when people use roads.

This is not about entrenched positions, it is about understanding how reports have been prepared and their validity.
Eric Bridgstock, Independent Road Safety Research, St Albans

And yet, you suggest that it "proves nothing and is not an appropriate report for this study".

The problem is that you - and many others on both sides of the 20mph debate - appear to hold completely entrenched views, and instinctively attempt to discredit any piece of work that does not support your entrenched position.

You are far from the only person to act in this way, but it does make meaningful discussion very difficult, if not impossible.

Having said that, we have the opportunity to hear from you - and you will have the chance to hear oposing views - in Harrogate next week. Can I therefore suggest that from here onwards we keep our powder dry until we meet face to face a week today.
Nick Rawlings, editor, Road Safety News

The London report contains “There is good evidence internationally for the effectiveness of reducing the speed and volume of traffic for reducing injury rates” and then goes on to employ models as to what might have been happening in London, without any verification or validation. The London report proves nothing and is not an appropriate report for this study, which is supposedly based on measured data.
Eric Bridgstock, Independent Road Safety Research, St Albans

Barring motorways and some lengths of dual carriageways all roads have residents. So what is a residential road? If one accepts that residential streets should have 20mph limits, at what point in housing density do you say to residents that they are not worth protecting? Perhaps Mr Rod king could answer that?

The 20mph speed limit was abandoned in 1930 as outdated, even for the primitive vehicles of the time, now we have thousands of miles of re-instated 20mph roads. Of course this speed applies through the night, when 99% of the population are asleep in their beds!
People no longer live/work in the same street, we want to travel and travel in reasonable journey times. We could be spending billions on HS2 but all the poor driver sees is speed downgrades and congestion as well as the strangling of our economic arteries. Frustrated/angry drivers do not make the best drivers!
Terry Hudson, Kent

Nick, Perhaps I should have more accurately aimed my comment. I did not refer to ‘your work’. My comment was relating to the inconclusive evidence as is available which is precisely what your work has shown and which you freely admit at the outset. And yet, there is a table of percentages listed and published that are based upon such data which has been described even by yourself as making analysis very difficult. I would go as far as to say almost impossible due to the individual nature of each and every accident. Yet those who would have traffic reduced to walking pace as it suits their ‘emotional’ claims and reactions, and which in their minds seems a most logical conclusion - pick up the near impossible and laud it as fact. So fragile. Seven areas are disregarded as not being focused on, yet without collation of precise and exacting data for relative and comparative conditions, there is no case to be addressed.

I have often said and will continue to say that road safety is an attitude of mind. Those whose minds are bent on speeding in busy thoroughfares will neither have such an attitude, nor will they heed any lowered speed limits. The 20mph speed limit will most likely be brought in regardless of the lack of accurate and available comparative data, for why should real data stop those with a desire to afflict other road users with the necessity of continually checking their speedometers instead of concentrating on the road.
Derek Reynolds

One or two people have referred to traffic diverting and reduced traffic volumes, however we're talking predominantly about residential areas and therefore by their very nature, people still have to go to and from their homes and local amenities and have visitors and deliveries etc, i.e it's all local traffic, so how would it be any less? Notwithstanding that, what sort of person anyway, on finding part of their usual route now with a reduced speed limit would seek out another, presumably less convenient route? I can't see that there would be any change at all in traffic numbers in these sort of areas and it shouldnt really be introduced into the equation.
Hugh Jones, Cheshire

Nice effort but, I agree, a bit crude. Did speeds actually fall and how many were speed related before and after? What about emissions? Nitrous oxides, carbon monoxide etc. At 20mph catalytic converters just don't work, as the Radcliffe Group demonstrated years ago. Did traffic divert? Were flows monitored?
Roger Harding

I see no evidence of bias in RoadSafetyGB's analysis of 20mph. They set a clear objective: changes in casualties/collisions after 20mph compared to neighbouring areas and have produced an excellent report. Every word within “What data did we find?” concurs with what I found and the conclusions are reasonable with the caveat that the evidence is “weak”.

The central issue highlighted seems to be that road safety engineering policies are being pursued at enormous cost on the basis of weak evidence. I can't think of any other area of safety engineering where that is allowed so shouldn't road safety adopt similar high standards?

The best quality evidence comes from running simple scientific trials called RCTs.
http://speedcamerareport.co.uk/02_scientific_trials.htm
We just restructure how 20mph is implemented and the RCTs would tell us the effect of 20mph directly. We wouldn't then need this perpetual debate over what effect 20mph might have had.
Dave Finney, Slough

When appraising any accident reduction/road safety intervention - whether it be engineering, enforcement or education - simply totting up before and after recorded accidents is not really going to tell us much. A lot more can be gleaned from knowing the where, how and why of each individual accident and whether it could have happened anyway, regardless of the intervention - in this case, the prevailing limit and compliance thereof.
Hugh Jones, Cheshire

This is a valuable and honest attempt to answer important questions about the impacts of 20mph schemes. The author has been quite open about the limitations of the data and the analysis and has called for more detailed investigation. I would hope that anyone with a genuine interest in improving road safety and reducing casualties would support this. Constructive criticism is fair enough and can be helpful but sniping about "preconceived ideas" smacks of preconceptions from an opposite perspective. The particular criticism about not measuring changes in traffic flows [and composition] could be applied to many casualty studies. Whilst these data would be interesting, their absence does not invalidate the analysis of changes in casualty numbers. The objective of the schemes was to reduce casualty numbers, not rates. (Accident migration is another matter.)
David Davies, London

I am more than happy for people to openly debate the merits, or otherwise, of this piece of work, but feel I have to correct two comments made below:

Derek - you say that my work is "as usual driven by emotion rather than statistics".
I can absolutely assure you that 'emotion' has played no part in this piece of work, and frankly cannot see what in the article might lead you to reach that conclusion.

Eric - you suggest that "the author has a pre-conceived idea as to what the outcome should be".
I can absolutely assure you that I had no pre-conceived idea of what the outcome would be. I approached the task with an open mind, which I still have.
Nick Rawlings, editor, Road Safety News

I am surprised that 19 readers think it acceptable that so much money has been spent on 20mph areas with so little competent analysis of the consequences. My own experience of looking at available data and claims is that they are based largely on wishful thinking and spin by vested interests. Nick is therefore right to caution Rod as he did, and right in calling, as I did, for competent, thorough and (in particular) independent monitoring of the data.
Idris Francis Fight Back with Facts Petersfield

How was data obtained? Did any authorities with known schemes fail to respond (perhaps because they had a “poor result”)? If so, there is a built in bias.

By not separating injury severities, the under-reporting of minor injuries hides any increases in serious injuries (as it did in Portsmouth).

Traffic volume not considered, nor the effect on neighbouring roads, so collisions and casualties routing around the 20mph area are not considered. Comparison with national trends is also relevant but ignored.

It is not acceptable to exclude the data for the year(s) between before and after, as it provides an indication of regression to the mean, as demonstrated by Finney, Francis el al. Speeds are not considered and yet typical results are a decrease in average speed of 1-2 mph, and an increase in some roads. It is inconceivable that such a change could lead to reductions in collisions/casualties of 10/20/30/40%.

“While not suggesting that the 20mph schemes are the only reason for these reductions, it is perhaps reasonable to assume they have played a part in this”. This is wishful thinking, suggesting the author has a pre-conceived idea as to what the outcome should be. Note that in half of the areas, there is no indication that casualty reduction in 20mph is better than the wider area.
There needs to be a logical and clear reason why an intervention has a (positive) effect. That is not the case for 20mph, especially when pedestrians are encouraged to feel safer and take less care when walking into the road (and vehicles are travelling at much the same speeds as when the limit was 30mph).
Eric Bridgstock, Independent Road Safety Research, St Albans

Such fragile data. Incomplete and as usual driven by emotion rather than statistics. Amongst those statistics there needs to be a definitive number of individuals who perform the same acts – crossing the road or driving – in the same places for a similar period of time both before and after any placed and enforced limit. How does one know that drivers have reduced their speeds? And what was the cause? How much has traffic volume in general contributed, road works, diversions, and how much have people begun to realise that more care needs to be taken in crossing or driving the roads? Such claims of 20’s plenty will never be able to be allocated simply to a speed limit, because the data will never be available to quantify their claims.

It’s all about accurate information. Without it, we are left with emotive responses to perceived problems.
Derek Reynolds

I agree with you. For some time we have been lobbying for central government to conduct proper analysis of road casualties across types of roads including the length of roads at different speed limits. Whilst thousands of miles of 30mph roads are being converted to 20mph limits each year the DfT stats still do not take account of or even track the length of roads at different limits.

Whilst central government is being "nudged" by traffic authorities to enable more cost effective implementation of 20mph limits (hence the recent revisions in guidance) it really is up to central government to accept that 20mph is already the preferred standard for most of our major conurbations and needs to respond with :-
1) Better analysis of casualty, liveability, and modal shift
2) Relaxed signage as 20mph becomes the 'norm' rather than the exception
3) Public engagement at a national level to complement the local work done by traffic authorities (see recent York article)

You say that "in almost every case" officers and members "have concluded that the 20mph limits were a positive move to support casualty reduction".

However, I feel I should point out that two of our key conclusions are that "there is an urgent need for a national initiative to record and report in a uniform way data relating to 20mph schemes" and that "the lack of clearly-presented data makes it very difficult to conclusively assess the impact of 20mph schemes with regard to casualties and collisions".
Nick Rawlings, editor, Road Safety News

I welcome this report. As a top level analysis it does what it says on the tin.

Of course at Traffic Authority level then each of these surveys have been analysed in detail by the respective officers and members. In doing so they have taken into account local anomalies and trends and in almost every case have concluded that the 20mph limits were a positive move to support casualty reduction. As a result those authorities and others are quite rightly expanding their use of this important initiative.
Rod King, 20's Plenty for Us

It is disgraceful that so much has been spent without consistent or competent analysis of results, or indeed any plans for to do so.

Welcome as it raising the issue, this analysis is however flawed. It fails to differentiate between minor injuries and KSI, concentrates on collisions instead of what really matters (casualties), ignores traffic diversions that relocate but do not cut numbers and reviews together very different sign-only "areas" with traffic-calmed "zones".

in August Oxford's claims of success ignored that the reduction in all collisions was little different from national trends and that unchanged KSI coincided with 30% national falls (similar to Portsmouth's 4 year results of KSI some 50% higher than national trend (see my web site). There is far from enough reliable speed or volume data available to relate to casualty numbers, but there is now ample casualty data. Surely it should be analysed properly, before spending yet more millions?
Idris Francis Fight Back with Facts Petersfield

I must congratulate RoadSafetyGB on excellent work. When this analysis was proposed I wondered if RoadSafetyGB knew the difficulty of their task and obviously each interested party will look at the results to justify their opinion to be confirmed.

There are further analysis problems as well. Site selection effects (such as RTM) have not been considered, neither have serious injuries and no separation of the effect of 20mph from humps and bumps but, as the report states, the quality of evidence is too poor for even basic analysis.

If all the current schemes had been installed within scientific trials, not only could we be confident of the effects of spending £millions on 20mph, but all the results could be combined into one overall convincing result.

The failure to run scientific trials has been a missed opportunity. Let's not allow new 20mph schemes to miss out as well.
Dave Finney, Slough

This is a desk top review of the available information - it does not claim to be an in depth statistical analysis. The exclusion of traffic flow data is listed as one of the issues specifically not included within this review.

Conclusions 1 and 2 make the need for better information very clear. Conclusions 3 and 4 are made within the context of the available evidence and are put forward as "reasonable" conclusions. If people have diverted their journeys to avoid the zone or limit, the effect on that area is unchanged.
Honor Byford, North Yorkshire

An interesting, but completely invalid statistical analysis I'm afraid. I see no figures for the change in traffic volumes before and after the introduction of these 20 zones. If there is less traffic because people are finding alternative routes to miss the zones then it stands to reason that the number of casualties will be lower as a result. These figures would only be valid if there was no change in the traffic volumes.
Duncan MacKillop, Stratford on Avon